1. Technical Field
The present invention relates to a cylindrical shaft, a transport roller, a transport unit and a printing apparatus.
2. Related Art
Up to now, various printers have been provided as a printing apparatus. Such printers are adapted to transport a recording medium, such as paper or the like, to a printing unit by a transport roller (a paper feed roller) and a driven roller, print the recoding medium, and then eject the recording medium by a paper ejection roller (a driving roller) and a driven roller (a knurled roller).
In this printer, the paper is pinched between the transport roller and the driven roller, and if the transport roller is rotated in that state, the paper is moved in a sub-scanning direction perpendicular to a moving direction of a carriage. Accordingly, the paper is transported to a recording position with high precision, and is sequentially sent according to printing speed. Therefore, high transport power is required.
In order to enable the transport roller to sustain high frictional force, Japanese Patent No. 3,271,048 discloses a technique of forming a plurality of bosses on a circumference of a metal rod by perforation processing.
However, since the bosses are formed on the surface of a shaft shape (a cylindrical column shape) along a circumferential direction, there is a problem in that the workability is bad. Further, there is another problem in that cost is increased due to the use of a solid material.
Under these circumstances, JP-A-2006-289496 discloses a shaft (a cylindrical shaft) of a cylindrical shape (a hollow shape) formed by bending a metal sheet which is used as a solid metal rod member, for the purpose of cost reduction.
That is, the cylindrical shaft formed by a metal sheet, of which a pair of opposite ends are joined, is provided in JP-A-2006-289496, each end including a convex portion protruding from the end and having a portion of which a width is widened as it becomes more distant from the end, as well as a concave portion depressed from the end and having a portion of which a width is widened as it becomes more distant from the end. The convex portion and the concave portion of the end at one side are fitted to the concave portion and the convex portion of the end at the other side.
However, in the case in which the cylindrical shaft, as shown in JP-A-2006-289496, is applied to a transport roller in order to promote reduction in cost or weight, it is difficult to provide the transport roller with high frictional force.
For example, it is considered that the surface is punched by pressing process to form the bosses shown in Japanese Patent No. 3,271,048; however, there is a problem in this case in that it is likely to deform because of the hollow.
Further, in the cylindrical shaft disclosed in JP-A-2006-289496, the pair of opposite ends are provided with the convex portions and the concave portions which are formed along the entire region (the whole length) in a longitudinal direction thereof, the convex portion and the concave portion of the pair of ends being respectively fitted into each other. That is, a joint between the pair of ends is provided with a fitting portion of the convex portion and the concave portion continuously formed over the entire region (the whole length) in a longitudinal direction thereof.
However, if the fitting portion by a concavity and a convexity is formed on the entire region or partial region of the joint, it is difficult to fit the concave-convex portion at the fitting portion as the design. For example, it is difficult to place a leading end of the convex portion near the corresponding concave portion in plane, without unevenness between the convex portion and the concave portion. As a result, deformation or distortion is likely to occur over the entire cylindrical shaft, and it is difficult to obtain good precision regarding the shape or dimension, for example to obtain circularity or tremor of above a predetermined value.
As described above, if the cylindrical shaft having insufficient precision regarding the shape or dimensions is applied to the transport roller, it plays a role in causing transport irregularity. Further, in a so-called ink jet printer which ejects fin droplets to form an image, since it requires very high paper feeding precision, the occurrence of the transport irregularity becomes an especially significant problem.